Study On The Multi-parameter Measurement Of WDM/TDM Fiber Optic Sensing Networks

The development trends towards next generation of optical fiber sensing network are huge capacity,long distance and high sensitivity.With the further development of internet of things(IoT),the requirement of huge capacity of optical fiber sensing network becomes more urgent.The huge capacity of optical fiber sensing network includes two aspects of content,more points and more parameters of measurement.In 2013,the National Engineering Laboratory for Fiber Optic Sensing Technologies in Wuhan University of Technology,has successfully developed on-line wrote no-weld friber Bragg grating(FBG)array.It made the suitable sensing cell and got rid of high loss,low strength in trandional series connection method in the process of large-scale FBG network.Then,the demodulation set for time division multiplexing(TDM)and wavelength division multiplexing(WDM)fiber sensing network was developed with the ability to mulplexing thousands of low reflectivityFBG along a single fiber.The low reflectivity FBG array has been becming one of the most promising direction for huge capacity FBG sensing network.With this background,it is the time to develop multi-parameter measurement based on low reflectivity FBG.The paper mainly studied the multi-parameter measurement technology for low reflectivity FBG sensing array.With the aim to solve key problems of mulit-parameter low reflectivity FBG array based on sensitive materials,this work investigated two typical cases of multi-parameter technology,hydrogen sensor array and relative humidity and salty sensor array.Firstly investigation was conducted on the method for fabrication of hydrogen sensor array and relative humidity and salty sensor array based on WDM/TDM fiber sensing array.Further research work concentrated on the low refletivity FBG sensing theory based on sensitive materials,fabrication and performance optimizing.Then the measurement system and performance evoluation was studied for multi-parameter WDM/TDM fiber sensing array based on sensitive materials.Finally it realized measuring temperature,humidity,hydrogen concentration and salty along the same fiber.The main contents and obtained results are listed as following.(1)Study on the simulation of the sensing characteristics of WDM/TDM fiber sensing array.Combining coupled mode theory and calculation method,the reflected spectrum expression of low reflectivity FBG was deduced.The attenuation of multiplexing different reflectivity FBGs in one fiber was simulated and experimentally.(2)The spectrum and electrical signal modulation characterication of WDM/TDM fiber sensing array were studied.Combing the theory of hydrogen sensor response including metal film and chemical film,the fabrication of low reflectivity FBG hydrogen sensor array was proposed.The FBG hydrogen concentration sensor array based on Pd film was fabricated,and experiments verified on WDM/TDM fiber sensing array when the hydrogen concentration in the range of 0%~3.6%.(3)Study on the mathematical model of water absorption properties of polymide humidity sensitive material,and the expression of polymide low reflectivity FBG humidy sensing was dedued.Two kinds of fabrication tehchnogy of humidity sensing array based on polymide,including lifting and coating method,were studied.WDM/TDM humidity fiber sensing array was fabricated based on lifting method.Experiments verified the sensitivity of humidity sensor was 1.134-1.832pm/%RH when relative humidity was in the range of 23.8%RH-83.4%RH.Then the affect of the thickness of humidity sensitive film to the performance of humidity sensor was analysed.(4)The WDM/TDM fiber sensing array with doping of carbonyl iron powder for salty sensing was studied,experiment verified and analyzed the sensor performance.(5)Study on the fabrication of multi-parameter WDM/TDM fiber sensing array on sensitive materials.It experimentes were conducted on a FBG sensor array with the reflectivity of 0.04% along one fiber.It could measure temperature,humidity,salty,hydrogen concentration at the same time by the same demodulation system.The multi-point and multi-parameter sensing network based on WDM/TDM fiber sensing array was built.The measurement system for sensing array was designed and the system performance was studied from the aspect of capacity.